Surface engineering of Ti and low-Al content Ti-base alloys for high-temperature environmental protection

An oxygen barrier coating has been developed, which is effective in preventing oxidation and oxygen embrittlement of Ti and several low-Al content Ti-base alloys during exposure to oxidizing environments at elevated temperatures. The fabrication process has involved magnetron co-sputtering of titanium and aluminum followed by vacuum annealing and plasma immersion ion implantation of fluorine. The resulting coating consists primarily of γ-TiAl while containing a minor portion of Ti3Al present in the coating/substrate interfacial region. The implantation of fluorine provides the necessary conditions for triggering the halogen effect upon subsequent high-temperature exposure in air. Systematic characterization of the coating material in terms of microstructure, phase formation and element depth distribution has been performed using cross-sectional transmission electron microscopy (XTEM) in conjunction with electron energy loss spectroscopy (EELS), energy-dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), and elastic recoil detection (ERD). Overall, the coating fabrication process has been found to be independent of the substrate material making the technique universally applicable at least to the alloys studied. Following oxidation in air at 600°C for 100 h, specimens have been prepared for metallographic analysis, and their cross sections have been characterized by scanning electron microscopy (SEM) in combination with EDX, and electron probe microanalysis (EPMA). The results obtained show that during oxidation exposure the γ-TiAl coating is capable of forming a protective alumina-containing scale, which serves as an oxygen barrier, thereby preventing oxygen embrittlement in the substrate material. In addition, since the only constituents of the coating are Ti and Al, it exhibits excellent chemical substrate compatibility.